Configuration control in multiprocessors



y 1968 T. s. STAFFORD ETAL 3,386,082

CONFIGURATION CONTROL IN MULTIPROCESSORS 12 Sheets-Sheet 5 FIG. 6 smus Filed June 01 4 E 5 c 0 2 I A w n [I P 3 our a EN MED 1d P C 2 M 0 1. C 2 E H E S m c 9 P 2 R 00 c 7 0 ET B N G \I MM 5 ww 4 SE 5 2 0 .1 9 P 1 4 N) 00 7d C 2 2 Kw FIG. 7

COR SE IOCE CCR IOCE FIG.8

CCR T CU FIG.9

IOCE

May 28, 1968 T. s. STAFFORD ETAL 3,386,082

CONFIGURATION CONTROL IN MULTIPROCESSORS Filed June 2, 1966 12 Sheets-Sheet 8 SOON/CE aacuunc. RESPONSE (SE v v u OR 333 FIG. 12

320 OR CHI/0E1 AND ANDS [IN/[IE2 SM/CEg/BIT SE1 5 CCR/SEHBIT 502 AND ANDS 502 50s 36 smczysn SE1 5e CCR/SE1/BiT 5C5 CM/CE4 AND SM/CEq/BIT SE1 \305 CCRISEUBIT s0 SE1 TEST LATCH OFF SE4 TEST LATCH 0N SE1 KEYS 5 SC P P BYPASS 4 s1 1 2 CIRCUIT SE1 TEST swncu on 56 L TEST W OCR/SE1 OUTPUT T0 55 G RECEWING GATES SE1 5n 5&6 TEST LATCH 0N OFF May 28, 1968 T. S. STAFFORD ETAL CONFIGURATION CONTROL IN MULTIPROCESSORS Filed June 2, 1965 12 Sheets-Sheet 9 SOON/GEM RECONFIG. RESPONSE (101) OR FIG. 13

OR [HA/0E1 SM/CE /BlT 10 1 36 ccR/Im/Bn s0 wAND ANDS cum? SM/CE /BIT 10 3 g 55 CCR/IOUBIT s0 AND ANDS cum b-D SM/CE /BIT 10, WW4 CCR/lOq BIT 505 AND ANDS as SM/CEq/BIT 10 as? CCR/IU1IBITSC4 AND 7 ANDS [0 TEST won OFF 10 TEST LATCH ON 56 101K vs oRs ANDS n f M 55100 MET A sea/10 s s" P 5E P SE c5 P/ 1\21l2|5l4%11\2[5j45|e]1[a29110|11\121\2\3\45W YflQ TJ 52 a 10s 4 BYPASS 4 1 2 CIRCUIT 0 TEST swsrcu on 56 ANo- 10 15s? svmcn OFF r- OCR/101 OUTPUT 101 TEST men on on May 28, 1968 Filed June 2, 1965 T. s. STAFFORD ETAL 3,386,082

CONFIGURATION CONTROL IN MULTIPROCESSORS 12 Sheets-Sheet 1 0 FIG. 14 INTERNAL OPERATIONS PERFORMED BY CE,UNDER SE PRG CONTROL, IN CONNECTION WITH RECONFIGURATION INITIATE PROGRAM INTERRUPTION IN RESPONSE TO ELC OR SUPERVISOR CALL INSTRUCTION ENCOUNTERED IN CURRENT PROGRAM COLLECT CURRENT PROGRAM STATUS WORD (PSWI FROM LOCAL BUFFER REGISTERS AND STORE AT UNIOUE LOCATION IN A PREDETERMINED SE FETCH NEW PSW FROM ANOTHER UNIOUE LOCATION IN SE ACCORDING TO CLASS OF INTERRUPTION DISTRIBUTE NEW PSW TD LOCAL BUFFER REGISTERS AND PROCEED TO FIRST INSTRUCTION OF NEW PROGRAM AT SE ADDRESS REFERENCED IN NEW PSW INVESTIGATE ELEMENT CCR STATES VIA STATUS TABLE STORED AT PREOETERMINED ADDRESSES IN SE INCOMPLETE PROCEED CONFIGURATION COMPLETE EXIT FROM SELECTED ADDRESS IN SE TO LOCAL BUFFERS; VERIFY SUPV. MODE BIT IN PSW; CHECK CM IN RI FOR PERMISSIBLE SC BIT CONFIGURATION; CHECK OWN OCR/OE STATUS BITS AND OWN CE TEST SWITCH; SET SCON LATCH SIMULTANEOUSLY TRANSFER CM FROM RI AND SM/BIT CE FROM R2 TO OWN CCR RECEIVING GATES IF OWN STATUS BITS ARE 00 OR II, AND TO ALL OTHER OCR RECEIVING GATES IF, IN ADDITION, OWN CE TEST SWITCH IS OFF UPDATE STATUS BITS OF RESPONDING ELEMENTS IN STATUS TABLE y 1968 T. s. STAFFORD ETAL 3,386,082

CONFIGURATION CONTROL IN MULTIPROCESSORS Filed June 2, 1.965 12 h ets-Sheet 1:":

FIG. 16

ELEMENT CCR/BIT S CCR/BIT S PAM2 1 1 PAM 1 1 TCU; 1 1

United States Patent 0 3,386,082 CONFIGURATION CONTROL IN MULTIPROCESSORS Thomas S. Stallord, Wappingers Falls, Donald C. Burnstine, Hyde Park, Gerard T. Paul, Poughkeepsie, and John R. Rogaski, Woodstock, N.Y., assignors to International Business Machines Corporation, Armonk, N .Y., a corporation of New York Filed June 2, 1965, Ser. No. 460,776 17 Claims. (Cl. 340-172.5)

ABSTRACT OF THE DISCLOSURE The present multiprocessor apparatus includes, in effect, a distributed interconnection system such that failure of a portion of the interconnection system does not completely disable the apparatus. Each element-of a plurality of computing elements, a plurality of storage elements, and a plurality of other information processing elements of the total multiprocessor apparatusis equipped with an individual Configuration Control Register for selectively controlling the flow of information between the respective element and other elements of the apparatus. For controlling the interconnection system represented by the Configuration Control Registers, with the redundancy necessary for reliable fail-safe" operation of the system, each of a plurality of computing elements is provided with means for independently developing, selecting and conditioning signals and with means for broadcasting these signa s to all elements of the apparatus, including the originating element. The selecting signals are utilized at the receiving elements to selectively limit application of the conditioning signals only to Configuration Control Registers of elements designated by the selecting signals, such elements having been predetermined by the o iginating computing element. The conditioning signals serve to condition the Control Registers of the selected elements to desired states of information flow control also predetermined by the originating computing clement.

The invention hereof pertains to the control of interconnections between independent elements of a multiprocessing complex. In particular, it pertains to a system of connection controls which can be used to reliably maintain continuity of data processing operations and cllicicnl distribution of element work load assignments within a multiprocessing complex in a variety of problem situations.

In certain real-time data processing situations for example in the processing of air tratfic control data, it is neces y to maintain substantial continuity of operation in order to avoid loss of data when circuit failures or other causes of processing bottlenecks are imminent or have actually occurred. It is conventional in this regard to provide duplicate system elements which may be maintained available, on a standby or other basis, to be switched into an active processing system as a substitute tor a failing element or to back up an element which is being overloaded with two many tasks. The weak link in this procedure has been the means used to accomplish the switching in of the back-up element and, if need be, the disconnection of the failing element. Hitherto, the practice has been to provide a. centralized arrangement of master switching controls which, in response to signals indicating an element failure, or other condition requiring attention can act to switch in back-up elements so as to reorganize the processing system, or systems, then in ope ation. One problem associated with this form of control is that the master switching controls themselves are 3,385,li32 Patented May 28, 1968 susceptible to failures capable of interrupting operations and causing irreparable loss of both data and data processing time.

It is, of course, possible to provide for manual disconnection of the master controls and the inter-element switches when a failure occurs. However, when time is of the essence, manual control is inadequate since seconds, or even minutes, might elapse between the time of occurrence of a failure and the disconnection of the master controls, during which time thousands of misrouted data transfers may occur with consequent loss of much data and disruption of data processing operations currently in progress.

Accordingly, an object hereof is to provide an improved system for controlling the configuration of interconnections between elements of a multiprocessing complex.

Another object is to provide a system for controlling interconnections between elements of a multiprocessing complex which can be adapted-to selectively reorganize interconnections between elements to maintain processing operations within the complex at a prescribed level of clliciency, continuity and reliability.

Another object is to provide a system for efiicienlly controlling interconnections between elements of a multiprocessing complex which can be expected to effectively maintain continuity of data processing operation during ordinary system failures.

Yet another object is to provide a system for efficiently controlling interconnections between elements of a multiprocessing complex which can be expected to reliably maintain continuity of processing operation during malfunctioning of any element of the complex including an element which can exercise supervisory control over all interconnections within the complex.

Another object is to provide a system for efficiently controlling interconnections between elements of a multiprocessing system including check features which precludc erroneous initiation of a change in interconnections or undcrisable acquiescence by an element in a proffered change.

A feature of the subject configaration control system resides in the provision. within each of a plurality of computing elements of a multiprocessing complex. of means capable. under prescribed conditions, of effecting changes in the states of receptiveness of any elements of the complex to signals transmitted thereto from other elements of the complex. Subject to certain restrictions any computing element can transmit configuration control signals which are selectively received by the other elements of the complex and determine the states of receptivcncss of the recciving elements. It is thus unnecessary to disconnect power from a faulty element to render other elements insensitive to it and by appropriately configuring states of receptiveness, the processing tasks of a faulty element can be reassigned to a health standby element, or added to the duties of an actively functioning element. Since such configuring can be accomplished automatically in response to a fault indicating s" at it is possible to maintain continuity of processing when a failure occurs. It is even possible to allow a faulty element to continue to transmit signals ex tcrnally while it is undergoing diagnostic testing or other maintenance procedures without risk of disrupting data processing operations currently in progress. Of perhaps equal significance, processing bottlenecks, due to assignment of too many tasks to one computing element can be anticipated and another computing element can be programmed to automatically step in and reconfigure the system so as to relieve the overloaded clement while the latter remains free to continue its necessary activities without interruption.

Annlltcr feature herein is that internal to each element of the complex there is provided a configuration control register (abbreviated CCR) having output control connections to gates which receive and pass signals transmitted by other elements. Thus, the receptivcness of an element to transmissions of other elements is a function of the present state of its CCR. The inputs of any CCR, subjcct to the output states of certain bits therein, are ZtCCCIsall lC to each computing element of the complex. Thus the content of any CCR, including a computing clement (LR. may be selectively changed by any one of the computing elements.

Another feature is that each element can transmit an element check signal (ELC) in response to which the computing elements can initiate a reconfiguration program leading to modification of selected CCRs and consequent reorganization of the complex. In response to an I'LLC those computing elements enabled by bits in their respective CCR's to receive from the element transmitting the ELC select one of their number to issue a program of reconfiguration signals. These signals establish new information settings in selected CCRs, including, if need be, the CCR associated with the element issuing the ELC signal. The issuance of an ELC signal may be conditioned upon the results of parity cheeks, power supply cheeks. logic checks, work load checks or checks of any other element condition which is stuiiciently pertinent to system operations to warrant consideration of a reconfiguration program.

The program instruction executed by a computing element to reorganize element system assignments within the complex-Le. to modify selected (TCltsis referred to herein as SCON (abbreviation for "Set Conlignratitm" t. Programs containing such instructions are stratcgicatly located in a plurality of storage elements of the complex on a redundant basis so that failure of any one storage element will not prevent reconfiguration of the complex.

In response to an element check or other condition calling for possible reconfiguration of a system, one of the enabled computing elements determines the condition of the complex, and if circumstances warrant further action, it retrieves one or more appropriate SCON instructions from an associated storage element and executes them. Under the direction of the selected SCON instruction, the computing element issues a reconfiguration signal, known as a configuration mask (CM), and an element selection signal. or selection mask (SM). The CM signals are selectively accepted as new CCR entries by the elements designated by SM.

The foregoing and other objects and features of the invention will be more fully understood and appreciated upon consideration of the following description thereof taken with reference to the accompanying drawings wherein:

FIGS. l---'t, when arranged according to the plan shown in FIG. 5, constitute a schematic illustration on a general level of a multiprocessing complex including a configuration control system organized in accordance with the present teachings;

FIGS. 69, inclusive, provide diagrammatic illustrations of the format of the configuration mask information stored in the configuration control registers (CCR's) associated with the different types of elements shown in the complex of H08. 1-4;

FIG. is a schematic of a representative computing element configuration control register (CCR/CB and access controls therefor;

FIG. it is a schematic drawing illustrating parts of a representative computing element (IE which can control the acquisition and execution of SCON instructions for reorganizing the multiprocessing complex hereof;

FIG. 12 is a schematic illustration of pertinent parts of a representative storage element configuration control register (CCR/SE and associated access controls therefor;

FlG. l3 is a schematic of pertinent parts of a representative input/output control element configuration con trol register (CCR/IOCE and associated access controls; FIG. 14 is a schematic flow diagram of a sequence of operations executed internally by a computing element in connection with the handling of a SCON instruction;

FIG. 15 contains a table which defines the operational capabilites of each element with reference to certain status bits (S) contained in the CCR internal to the element; FIG. 16 illustrates a typical stored status table listing the current status bits of all element CCRs of a complex. This table may be used as a stored reference by a CE in connection with the Instruction Fetching routine to determine its selection of a SCON instruction for execution.

GENERAL DESCRIPTION As shown in FIGS. 1-4, a muliprocessing complex of the type under consideration herein typically may include a plurality of computing elements (CE), shown at 1 and 2 in FIG. I, and may include a plurality of other specialized elements, such as storage elements (SE), shown at 3 and 4 in FIG. 2, input-output control elements (IOCE or 10) shown at 5 and 6 in FIG. 3, and peripheral adapter elements 7, 8 (FIG. 4).

The adapter elements are each connected to a plurality of peripheral devices such as tape stores, communication terminals, printing devices. and the like. The tape store adapter units are hereinafter designated by the symbol TCU (abbreviation for tape control unit) and other peripheral device adapters are designated by the letters PAM (abbreviation for peripheral adapter module).

As may be inferred from the descriptive terminology used above, CEs perform processing operations on internal programs and information supplied by SE's and lOCEs provide a link for exchanging information between the SEs and external input and output devices via the additional buffering afforded by the adapter elements.

Each element is provided with a configuration control register 10 (abbreviated CCR), two sets of receiving gates 11, 12 and other internal components 13 collectively denoted other parts. These other parts may include individual power supplies. To simplify FIGS. 1-4, only the internal organizations of representative elements, CE SE IOCE and TCU are shown. It should be understood that the internal organizations of other elements in each category would be similar to those of the representative elements.

The receiving gates 11 control the introduction of new information settings into the respective CCRs, and the receiving gates 12 control the reception of other information by the other internal circuits 13 of the respective elements. The receiving gates 11 and 12 of each element are controlled by various digit outputs of the associated CCR as indicated by lines 15, 16 and 17. Thus, all incoming information signals proffered to an element can be selectively received or rejected by the element in accordance with the contents of its associated CCR. An exccp tion to this is found in the IOCE elements, Information proffered to an IOCE 5, 6 by a peripheral adapter unit 7, 8 is permitted to bypass the receiving gates 12 cOntrolled by CCR/IOCE. This is because there are other internal receiving gates in each IOCE, not controlled by CCR/ IOCE, which nevertheless prevent unauthorized reception of information transmitted from an adapter.

The other parts 13 of each SE, 3, 4, include a fast access store, such as a magnetic core matrix memory. and access controls for reading and writing information relative thereto.

The other parts" 13 of each CE include computing and control circuits, for operating upon stored program instructions and data stored in an SE, for executing SCON instructions and selectively issuing configuration control information to the CCRs, and for handling the acknowledging responses of elements intended to have their CCRs reconfigured by such information.

The other parts" of each IOCE include one or more channel systems for selectively conveying information bidirectionally between peripheral device adapters (TCU, PAM) and SEs, according to the needs of the CEs.

Data and control signals not directly connected with configuration control are exchanged between 21 CE and an SF. via group bus 20, one of the tributary buses 21 (FIG. l) or 22 (FIG. 2) feeding into bus 20, and one of the branch buses 23 (FIG. 2) or 24 (FIG. 1 extending from the group bus.

Data and control signals not directly connected with configuration control are handled between CE and IOCF units via group bus 25 (FIGS. 1 and 3). one of the tributary buses 26 (FIG. 1) or 27 (FIG. 3) feeding into group bus 25, and one of the branch buses 28 (FIG. 3) or 29 (FIG. 1) respectively, extending out of the group bus.

Data and control signals not directly connected with configuration control are handled between SE and IOCE units via group bus 35, one of the tributary buses 36 (FIG. 2) or 37 (FIG. 3), and one of the respective branch buses 38 (FIG. 3) or 39 (FIG. 2).

One other path for the handling of data and control signals is that between the IOCE units (FIG. 3) and peripheral adapter units (FIG. 4). It includes paths defined by group bus 45, one of the tribuary buses 46 (FIG. 3) or 47 (FIG. 4), and a respective branch bus 48 (FIG. 4) or 49 (FIG. 3).

Each element of a group of elements having tribuary buses feeding a group bus can transmit signals selectively to any element of a group having branch buses extending from the same group bus. Thus, any of the m computing elements, CE can selectively send information to and receive information from any one of the n storage elements SE via group bus 20 and appropriate ones of the tributary and branch buses 21-24. Hereafter, the foregoing connections will be referred to as data paths to distinguish them from configuration control information handling connections between CEs and CCRs, referred to as configuration control paths." The latter paths are described in greater detail below.

Means not shown are provided for resolving conflicts created by conflicting calls for service by two or more elements from a single element: for example, simultaneous calls by two or more CEs for access to the same SE. Such means act to assign priority to a unique one of the calling elements according to a predetermined plan. Since this priority assignment plan is not pertinent to the configuration control system of the present invention it is deemed unnecessary to provide further details thereof in this discussion.

The following brief example will sufiice to complete the description of the data paths. Suppose CE, is engaged in executing a program stored in SE and wishes to retrieve its next instruction from an Instruction Address IA in SE CE, raises a call line, in a path 21. 20. 23, between CB and SE and transmits a representation of the address IA. along other lines in the same path. At some later time SE runs through a non-destructive read/regenerate cycle of access to internal address location IA. Upon readout of the contents of IA, SE raises a response line in the data path 22, 20, 24 between SE and C15 while at the same time transmitting the contents of IA to a special buffer register in CE, via other lines in the same path. The response signal is used by CE, to reset its call signal and the exchange is completed.

Signals participating in the establishment of new configuration control settings in CCR register 10 are handled through a configuration control bus system including a main group bus 60 (FIG. I), subgroup buses 61 (FIG. 2), 62 (FIG. 3), 63 (FIG. 4), and a plurality of tributary and branch buses and lines as described below. While any element may initiate a request (ELC) for action which may result in a new CCR setting, only a CE (FIG. I) can issue the actual CCR setting information, hereinafter termed configuration mask (abbreviated CM). Thus, each CF. is provided with a GM output bus such as the bus 71 ell) extending from (E (FIG. l), and CllL'll clclucul (t it is provided with a CM input bus 72 for calming; a CM from any CE to the receiving gates 11 of that CCR.

An abbreviated form of notation is employed herein to characterize the information units pertinent to a reconfiguration process. In general this notation has the form: A/B/C, where A represents the information unit, Fl rcprc sents the issuing source. and C specifies particular bits in A. Thus. the notation employed for example to designate particular hits a, b of a 36-bit CM word issued by (F would be CM/CE /IDIIS a, b.

As indicated above, a CM word issued by a CE unit via a CM output bus, such as 71 (FIG. I) would be applied in parallel to the CCR receiving gates ll of all elements in FIGS. 1-4 via respective CM branch bases 72 extending in parallel to all the inputs of such gates. Thus, the notation CM/Cih, at 72 indicates that all CM/Cl'i outputs extend as inputs via 72.

Those elements which are intended to receive an issued CM are designated by corresponding bits in a coincidentally issued Selection Mask word (SM). The latter is provided by the CE issuing CM on its associated SM CF. output has 73. Thus. the gates 11 of CCRtCF (FIG. 1). C(IlUSE (FIG. 2), CCR/IOCF, (FIG. 3). and CCR/TCU, (FIG. 4). are respectively selected by the conditions of bits designated bit CE bit SE bit 10 and bit TCU in an SM word. Since such SM words may issue from any of In source CE's, separate Sh t bit line is extended from the bus 73 of each CE to each grou of element CCR gates 11. Thus, the lines 74 extending to the gates 11 of CCR/CE in FIG. I can convey a CF, selecting bit of an SM word issued by any CE (including CB to the receiving gates 11 of C8,, as indicated by the notation SM/CE /bits (IE Similarly. the lines 7-i extending to SE (FIG. 2) convey SE selecting (its of SM words to the SE receiving gates 11 as in ate-l by the notation SM. CE ./Iiits 8B,. Likewise in Fit]. 3. the notation ShUCE bits IO adjacent bus "I -l indicates that the I0 selecting bits of all SM words arc conveyed to the gates 11 of lOCE Finally, in FIG. 4, SMTCF; bits TCU, indicates that a TCU selecting hit in each SM word is conveyed thereby to gates ll of TCLH.

It should be understood that the SM selecting hits are binary-valued signals which in dillerent binary states will tend to actuate or not actuate respective gates 11. It may further be inferred by noting the lines 15 and 1t extending from each CCR to the gates It that each ("CR can selectively accept or reject a proficred CM (i.c.. a CM accompanied by an active selecting bit of an 51M on a line 74) in accordance with the information currently stored in the (CR. The lines 15 issue from a pair of status bit stages (S S of each CCR. the combined states of which determine four element states: UL, ()1, It), and ll. The lines 16 issue from individual supcrvir-xory control hit stages (SC i of the respective CCR, and CUrlcSpOIltI individually to the different CF. elements of the complex. Signals on line 16 control acceptance of incoming (M's proffered by SM selecting bits of respective F. units. The status bits S determine whether the associated CCR is in a state permitting acceptance of a proffered CM. and if the state is such a permissive one. the control bits SC selectively determine the CH5 from which a CM may be accepted (i.e., the element selecting bits oi SM to which the gates 11 may react). An element accepting reconfiguration signals its acceptance of the protl'crcd CM to the issuing CF. via an individual response line 75. Failure to receive such a response from any element designated by SM is taken as an indication of program error. or other failure, by the issuing CF.

Over-riding control over the S and SC bits in each element CCR is afforded by reset (R) input lines 76 which are coupled to reset output lines 77 issuing from the CBS (FIG. 1) by manual reset controls 78, and by SC bypass circuits which are not shown in FIGS. l-4. The latter circuits which are considered in detail in connection with the discussion herein of FIG. It) are eilrctivc to force all SC bit outputs of CCR to the 1 condition when all SC bits are coincidentally set to O. The effect thus produced is the same as if all SC bit states in the CCR had been set to I. Were it not for this, if all SC bits in CCR became coincidentally set to 0 due to some faulty condition the CCR of the associated element would become isolated from further reconfiguration control.

Each element is further provided with an Element Check signalling line (ELC) 79 by means of which the existence therein of an error or other condition requiring attention is made known to the CEs. Recall that the CE states (S) are so maintained that at least two CE's will be able to respond to the ELC. Thus invariably one CE will respond to the ELC, execute SCON, assemble CM and SM, and thereby reconfigure the complex. In response to an ELC signal which is a pulse of short duration, subject to certain checks and priority resolution procedure described hereafter, one of the enabled CEs other than a CE originating an ELC, will perform a program routine to evaluate the cause of the error or failure which gave rise to the ELC signal. This one CE will then assemble and forward SM and CM words under control of SCON instructions through the bus system described above. These words condition the selected CCRs and thereby reconfigure the complex. Assuming that the SCON instructions are appropriately programmed the data processing operations in progress prior to the ELC may be continued without significant interruption.

The response and ELC output lines 75 and 79, of all elements are extended in parallel via branch buses 80 (FIG. 1) to the receiving gates 12 of each CE.

The general effects characteristic of a reconfiguration process as executed by the apparatus shown in FIGS. 1-4 may now be summarized as follows:

First, one or more CEs are made aware by an ELC or an internal condition, such as a decoded program instruction or manual switch setting, of a problem condition. After executing several internal checks to determine whether they can legitimately issue reconfiguration information, the CEs attempt to gain access to a predetermined one of several SEs containing information pertineat to reconfiguration. By executing a priority resolution procedure these CEs enable a unique one of their number to address the SE and withdraw therefrom a status table containing a listing of the status bits (S) currently stored in all CCRs of the complex. Utilizing this status intelligence the enabled CE exchanges other program information with an associated SE to determine a configuration of the complex suitable for maintaining continued processing. Branch decisions in this program guide the controls of the enabled CE to selectively address and execute one or more SCON (Set Configuration) instructions in SE appropriate to the circumstances. Each SCON instruction is retrieved and used by the enabled CE to control a corresponding internal subroutine during which appropriate CM and SM words are assembled in CE registers. Thereupon, the CM word is broadcast towards all elements via bus 60 and proffered to selected elements according to the coincidental states of corresponding selecting bits of SM. Those elements designated by SM which are permitted by the S and SC bits in their CCRs to accept the proffered CM do so and acknowledge their acceptance by a signal sent to the issuing CE via the response line 75. The response signal is utilized by means not shown in the issuing CE to reset the corresponding SM selecting bit in the CE register preloaded with the SM word.

When the mask issuing CE determines, by internal controls further considered herein. that all elements designated for reconfiguration by one or more SMs have accepted the correspondingly proffered CM, and therefore verified that the complex has been suitably reconfigured. it may terminate its execution of SCON instructions and resume operation as part of a processing subsystem within the complex, retrieving its next program instruction 8 from an SE to which it is configured by a corresponding SE bit in its CCR.

It will be noted that the output lines 17 of. each CCR control the receiving gates 12, of the associated elements, through which all information, other than configuration control information, or adapter information proffered to an IOCE, reach the internal parts 13 of the associated element. Thus, it will be understood that outputs of any CCR connecting to respective lines 17 control reception of information transmitted to the associated element from other elements and thereby the CCR contents in effect determine all signal flow between elements of the complex. A feature of this reception control is that a faulty element, even if it should become isolated from reconfiguration control and continue to transmit erroneous information, can still be cut off from all healthy elements of the complex by reconfiguration, i.e., by changing a bit in each hcalthy" elements CCR.

CC R FOR MATS Considering FIGS. 613, with reference to the complex shown in FIGS. 1-4, organizational details of the subject configuration control system may now be explained. FIGS. 6-9 show the relative formats of the control information stored in the respective CCRs of the CE, SE. IO, and adapter elements. The information in each CCR consists of selected parts of a basic 36-bit control word or CM format.

In the particular embodiment of a multiprocessing complcx described herein, there are provided four CEs, a maximum of 12 SEs, and a maximum of 3 IOCEs (i.e., [in-:4, ll, and p:3 Accordingly, each CCR con tains four supervisory control bit stages (SC the outputs of which control acceptance of a new CM proffered by corresponding ones of CE Each CCR also contains a pair of status bit stages (S and from one to four parity bit stages (P Each parity bit stage holds the anticipated correct parity of the 8 consecutive bits to the left thereof in the CCR and may be compared to a computed parity as a check on the correctness of those S-bits.

In CCR/CE (FIG. 6) there are 12 bit stages, designated CCR/CE /bits SE which have outputs controlling the receipt by the associated CE of intelligence forwarded by respective storage elements SE of the complex. There are also 4 bit stages CE controlling receipt by the particular CE of intelligence conveyed from correspondingly numbered CEs. and bit stages IOCE (or 10 7 for controlling acceptance by the particular CE of signals originated by corresponding IOCEs. The seven blank bit positions 90, distinguished by a pattern of parallel slanted lines. are considered spare or dont care positions since the outputs therefrom do not control receiving gates.

CCR/SE (FIG. 7), in addition to S, SC and P bit stages contain bit stages CE J, and 10 corresponding to the CE and I0 bit stages in CCR/CE (FIG. 6) for controlling acceptance by SE of signals conveyed from receptive CH5 and I'OCEs. All other bit positions are spare (unused). Since the eight positions 91 to the left of P are blank all of the nine stages 91 and P are unnecessary and may be omitted.

CCR/IOCE (FIG. 8) in addition to bit stages denoted S, SC and P. contain bit stages SE, and CE corresponding to correspondingly designated SE and CE bit stages in CCR/CE, for controlling acceptance by IOCE of signals conveyed from respective SEs and CEs. All other bit positions are spare. Since stage P, and the associated eight stages 92 are not used, all nine of the stages may be omitted.

The CCR of each peripheral adapter, for example, CCR/TCU (FIG. 9), in addition to S, SC and P bit stages contains bit stages [0 in positions corresponding to those held by the IO bits in CCR/CE. for controlling acceptance by the respective TCU of signals supplied by IOCE respectively. All other stages conmin unused or spare bits. Since the stages 93 and 94 respectively associated with P and P are unused, all eighteen of these stages are unnecessary and may be omitted.

Considering the CCR formats (FIGS. 6-9) in relation to the associated elements of the complex as characterized in FIGS. 1-4, a number of pertinent observations may be made. An element (for example an SE) may be prevented by a CE bit in its CCR from receiving ordinary communications from a particular CE and yet be permitted by an SC bit in its CCR to accept a CM proffered by the same CE. Secondly, since the CCRs of all elements have similar control information formatsfor example, the status and SC bits invariably occupy the first six register positions in all CCR registers, and CE bits, where available, are invariably located in the last four places of a group associated with parity stage P -a single pair of CM and SM words can be used to set a plurality of CCRs when circumstances permit (e.g., when the associated elements are grouped together in a subsystem). Thus, the network of gates and the programming required for entering information into a CCR may be simplified. Finally. it is repeated that while the adapter registers, such as CCR/ TCU, have IO bits to control reception of IOCE originated information, the IOCE CCRs do not contain adapter bits for controlling information fiow in the reverse direction. The reason for this is that the ordinary reciprocal controls between IOCE and adapter units are so arranged that if one cannot receive a communication from the other, it cannot provide a key sequence of responses necessary to actuate the other. It is, therefore, suflicient to provide CCR control in one direction only.

CCR/CE AND SETTING LOGIC Referring to FIG. 10, the configuration control register CCR/CE, of a representative computing element CB and the circuits controlling access thereto which are substantially identical in internal organization to similarly functioning circuits in the other CEs (C13 are seen to comprise detail features, as follows: For admitting new configuration control settings (CMs), proffered by any of the CBS including CE to the 36-stage register 100, which is the CCR/CE, associated with CB there are provided four AND circuits 101-104. The outputs of these control four respective groups of AND circuits 105-108. Each group of circuits 105-108 comprises 36 individual AND circuits for handling the individual bits of a CM signal presented to the group. Groups 105 to 108 are coupled to receive CM signals originating at respective CEs, CE, to CE. via respective groups of input wires 110 to 113. OR gates 1.15 combine respective outputs from each group of AND circuits 105-108 to provide a single 36-bit input path feeding into register 100.

AND circuits 101-104 are partially controlled by CE bits in selection mask signal words (SM) issued by respective CEs, CE; As an additional partial control, signals corresponding to the outputs of bit stages SC of register 100, except under conditions noted below, are applied to respective ones of circuits 101-104. As a third partial control, SCON enabling signals applied via line 119 to AND circuit 101, and SCON enabling signals transmitted by other CEs are applied via line 120 to the other three AND circuits 102-104. A fourth control input 121, to each AND circuit 101-104, is a TEST LATCH OFF signal provided by CE, control circuits shown in FIG. 11. This line is disabled only when the values of both status bits, and S in CCR/CE, are coincidentally 0 while a CB Test Switch (FIG. 11) is in an ON position, and it is restored to an enabled condition when the switch is thereafter placed in an OFF position.

An additional group of 36 inputs to OR circuits 115 not previously mentioned is supplied by lines 125. Signals on these lines correspond to respective manually keyed inputs 126 of 36 AND circuits indicated generally at 127, when the latter are enabled by a control signal on a common input line 129. Line 129 couples to the ON output of the TEST LATCH feeding line 121, so that the circuits 127 are enabled only when the line 121 becomes disabled; i.e., only when bits S and S in CCR/CE, are both 0 while the CE, Test Switch is ON. It is, therefore, clear that the contents of register can be manually changed through action of keys or switch levers only under certain predetermined status bit and test switch conditions, this feature serving as an interlock or precautionary measure precluding accidental isolation of an element from CH as a result of accidental manipulation of a CCR/CE, bit setting key.

The outputs of AND circuits 101-104 are also extended to OR circuit 133 which couples to the CE, configuration response line 75, also shown in FIG. 1. Upon application of a CE selecting bit of any selecting mask SM to one of the AND circuits 101-104, and translation of a signal through the circuit, a response is conveyed via line 75 to the CE issuing the SM. This response is used to reset the CB selecting bit in the register holding SM whereby the issuing CE is informed that CE, has accepted a proffered CM in CCR/CE When energized, reset line 135 sets each of bits SC to 1, and each statu bit 5,, S to 0, so that if the CE, test switch is OFF, AND circuits 101-104 will all be enabled, permitting any CE to reconfigure CCR/C13 A bypass circuit 137 translates the outputs of bit positions SC of register 100 to the output bus 138. This circuit includes four OR circuits 139-142. effective to di rectly translate respective SC; outputs of r gister 100, so long as one or more of these outputs are in the 1 state, and an OR circuit 143 and complementing circuit 144 providing an indirect second input to OR circuits 139-142. which is active whenever all bit stages SC, in register 100 simultaneously contain zeroes. AND circuits 101-104 cannot all be simultaneously disabled by CCR/CE /bits SC since the corresponding inputs are taken from the bypass circuit 137, and the four outputs of the latter invariably include a 1" signal. Thus, register 100 cannot he accidentally isolated from all CEs. As an additional feature, the circuits 137 produce an all ones output, upon detection of a CCR parity error, as suggested by input line 145 joining the output of complementing circuit 144.

Each of the other CEs, CE while not illustrated in detail should be understood to comprise CCR gates and controls corresponding in function and organization to those shown in FIG. 10 for CB except that the identities of inputs to the circuits corresponding to AND circuits 101-104 and 127 of CE, are different for each of the other CEs. For CE, and the AND circuits in the positions of AND circuits 101, 103 and 104 would be controlled jointly by CH TEST LATCH OFF and SCON signals originating at other CEs. and individually by hits in SM/CE SM/CE and SM/CE, respectively, and by bits SC SC and 5C from respective stages of CCR/C13 The AND circuits in the position of AND 102 would have inputs SCON/CB SM/CE /bit C15 and CCR/CE /bit SC;. The AND circuit corresponding to AND circuit 127 would be conditioned to pass C15 data key signa s by CE: TEST LATCH ON, and the output of the OR circuit corresponding to OR circuit 133 would represent the reconfiguration response of CB For CE; the AND circuits corresponding to 101, 102 and 104 would be controlled jointly by CE, TEST LATCH OFF and SCON signals produced by (E (F or CH, and individually by bits CB of SM/CE, SM/CE and SM/CE respectively. and outputs of bit stages SQ, SC and SC, respectively of CCR/CB The inputs to the circuit corresponding to AND circuit 103 would he SCON/C1 SM/CE /bit CH and CCR/CE /bit SC;,.. The inputs to the circuit corresponding to AND circuit 127 would be CE TEST LATCH ON and (IE DATA KEYS.

For CE the counterparts of AND 10], 102 and 103 would have joint inputs CE, TEST LATCH OFF and ISSUANCE OF SCON BY CE Con ideration is next given \iith reference to t' lt'tfi, It and 14 to the other internal parts of (E and by analogy to correspondingly func ioning parts in the other (i These include circuits which participate in the acquisition and execution oi SOON instructions and the collection and distribution of M and SM words to sclcetcd C(Rs throughout the complex with consequent r orgttnization of the associated elements within the conr plcx, The parts 160 are grouped into four main subdivisi ns designated receiving gates 161 ame as gates 12 i MG. 1), computing and parity checking circuits 163, interrupt controls 163 and instruction decoding and other controls 164. The latter includes CE Test Switch 165, other manual controls 165 and other electronic controls yet to be discussed The circuits 162 carry out computing and parity checking lunctions on information handled within 160 including checks on the information content of the configuration control register 100 ((CR/(EQ of FIG. ll). The circuits 164 provide all of the sequential control signals and include sulhcicnt locnl butler storage registers to enable (fi l; to carry out its compuing and data handling functions cliectively.

The other manual ct'introls 66 includes switchcs for controlling power sources. single cycle operation, and other functions requiring manual control. The controls 166 are effective only when a bistable test latch 167 is in the ON state as indicated by a predetermined signal level on line 168. Latch 167 is set to the ON state by an active output from AND circuit 169. The latter occurs only when test switch 165 is in the ON condition ant coincidentally an enabling output is received from AND circuit 170 in response to it indication from stages S; and S of (CR/(TE (FIG. l0). Once in the ON state latch 167 can be reset to OFF only by setting the te-t switch 165 to its Ol'l position. Thus, the bits S and S Cttfl re varied for test purposes while the test latch remains in the ON state.

The OFF and ON outputs of test latch 167 also extend to the lines E21 and 129 respectively shown in FIG. 10. Whenever the condition of the test latch is reversed from ON to OFF line 171 in FIG itl is energized to reset hit states S and S of (OCR/CE to (1, whereby CCR/CE; remains accessible to either manual rcct' nfignrntion by turning the test switch 165 lJZtCli to ON, Or by programmed reconfiguration as considered below.

in addition to the test latch there is provided a Si l (Set Configuration) enabling latch 175 which can be set to an ON condition by an output from AND circuit 176. Tircuit 1'76 is partially enabled by one si nal 177 of a group of sequential mieroprogrnms signals schematically represented at 173. These signals are derived duri lg n CE microprogram of operations culminating in the fetching to CE of a SCON instruction from one of the working storages Sil s as dc cri ed below. The signal :77 is invariably followed by a s nrtl l7) which cstablishc latch 175 in the OFF or RESET shite. In its ON stat latch 175 provides a 5("ON enabling signal to input line ll) oi AND circuit 10.! in l 'llc i. ill. It. in Iuhiition. tcst hitch [67 is OFF, a remote SfON enabling signttl is translated through AND circuit 180 to the receiving gates of all other element CCRs. In cttch Cli the remote Stir JN signals of the other CFi's are passed through an (1R circuit connection such as [t ll ti lti. ll? to it control line such as 12.0 lllU. lU

til)

Other conditions for completing the actuation of AND circuit 176 to set latch 175 are determined by the signal on line 134 (CCR/CE /bit SC and the outputs of OR circuits 135 and i86. The output of OR circuit is active if all of SC bits of the configuration task Chi/(TE; conveyed to bus 71 (FIG. 10) are not Simultaneously in the 0 state. The output of OR circuit 186 is active it either AND circuit 187 or AND circuit 176 is energi/ed or it microprogrnm control line 18) is excited. AND circuit 187 or 17%) will be enabled it the values of bits 5 and S; are both simultaneously equal to l or simultaneously 0, respectively. Thus, OR circuit 186 and AND circuits 87 and 17% together comprise (an inverse exclusive OR circuit acting upon the outputs of stage, S and Sf] (lf CCRNCEL Linc 189 is energized during manually initiated loading in an SE of a program scheduled for utilization by CB Hence, this line is identified by the symbol llL (abbreviation for Initial Program Loading Signal).

it should thcrel'ore be understood that SCON la'ch 175 is set to ON if microprogram signal E77 occurs while bit SC of OCR/"CF1 and at least one of the bits SC in CM/CE; are coincidentally in a 1 condition and either hits 5 and in CCR/CE are set to identical co:iclitions or control line 189 is excited. It should also be understood thttt latch 175 is rest to OFF by a signal on line 179 invariably following the signal 177. While in the ON condition latch 175 controls translation of SM selecting bits via line 119, FIG. 10, and via lines corresponding to 120 of HG. it) in all other elements.

The handling ot mask words CM and SM Within the circuits 16% is characterized schematically as follows. CM and SM are acquired by programming from SE units permitted access to 160 by gates 16L CM and SM are forwarded under microprogrnm control into selected registers and 196 respectively, which are designated R und R respectively. This is indicated s chcn'iti ically by re spcctivc broken lines 197 and 198. CM and SM are parity checlted through circuit 162 as indicated by broken lines 200 and 201, and also also coupled to outgoing buses corresponding to hoses 71 and 73 shown in FIG. 1. As previously indicated the positional format of the CM intelligcnce is the same or all elements with dill'erent bits masked upon entry into different clement CLTR's. Thu, Cl Cll CM contains status bits (S), supervisory control bits [SC], Sli hits, Cli hits, and [0 bits and parity bits in the positions shown in FIG. 6, but the element CCRs receiving a CM will receive selected portions of a C. M and dillcrcnt CMs may be sent out to ditlerent elements during a reconfiguration program, as will be described below.

The format of SM is indicated at 205. SM is a 36- bit selcction control word including eleven non-selecting bi s consisting of four ptarity bits P t and seven spare bit positions indicated at 297 and 208, and twenty-five selection control bits which are coupled to individual clement (ICR access gates such as AND circuit 191 in H6. 19, whereby selective translation to each CCR of :i simultaneously issued (TM may be ellcctcd. Thus, SM contains sit peripheral device adapter selecting bits 209, twelve Ct". selecting bits 21%. tour C(R/(fli electing bits 2, and three CCR/IO selecting bits 212. The values of these bits are 1 for selection and O for nonselection of the associated clement CCRs.

it would be helpful. at this point to consider the program and nzicropiogram processes by which a (E can reconfigure tie complex. Referring to llG. 14, this process is characterized in a nine step flow chart in which the sequential sub-program or nzicroprogram steps taken by a (E are numbered 225-233, and discussed below in that order.

Each of the CEs under consideration is designed to operate as an independent central processing unit (CPU) on instructions of the IBM Systciiz/Stll instruction set. this set of instructions and their implementation by a processor are characterized, for example, in copending patent application Ser. No. 357,372 of G. M. Amdahl et al., filed Apr. 6, 1964, which is assigned to the assignee of the present application. Thus, each CE is a processor having internal sets of controls for executing subprograrns or micro-programs required for interpretation and execution of instructions in the above set. It is further noted that each CE is provided with additional subprogram or microprogram controls for interpreting and executing a SCON instruction, as defined and characterized below, which is not a member of the IBM System/360 instruction set.

It is assumed that prior to step 225 (FIG. 14) the CCR organization of the complex of FIGS. 14 is determinable. This means that if the complex is first starting into operation it would be necessary to operate manual reset switches in all elements to establish initial states of receptiveness in the CCRs of the CES appropriate for initiating the reconfiguration sequence defined in FIG. 14. It is assumed that the complex is in operation prior to step 225 and a condition has occurred indicating possible need for reorganization.

In step 225 of FIG. 14, CE initiates a rnicroprogram interrupt sequence of a predetermined class having sensed a signal associated with that class of interruption. This may involve sensing of an element check request (ELC) issued by an element other than the sensing CE or a SUPERVISOR CALL instruction belonging to the above universal set. The CE begins its interruption sub-routine after completing its last previous operation. As part of this sub-routine the last instruction address and certain other items of information are assembled from local registers into a 64-bit program status word (PSW) which is stored (step 226) at a unique location in an SE with which the CE is then capable of communicating by virtue of the known status of reception control settings in the CCR/CE and CCR/SE registers. The unique PSW location corresponds to the type or class of interruption.

At step 227 a new PSW is fetched to CE from another unique location in that SE also corresponding to the class of interruption.

At 228 the new PSW is set up in local buffer registers of the CE and the CE proceeds to recover the first instruction of a new program series stored in an SE, in accordance with instruction address intelligence contained in the new PSW. In this new series the CE is directed (step 229) to test the status bits of all elements of the complex, by referring to a status bit table in a predetermined series of SE locations. This table is shown in FIG. 16 and discussed below with reference to that figure. Depending on the present status of the complex and the particular type of condition which caused the interruption of step 225 program branches are taken to direct the CE to either terminate the current program series (exit 237) and begin an ordinary program sequence or to continue in sequence to address a SCON reconfiguration instruction located in SE.

Assuming that the sequence leading to step 230 is followed, a SCON instruction is fetched from SE to local buffers of CE and the PSW in CE is checked to assure that the supervisory mode bit is appropriately set. The SCON instruction has an 18-bit format, parts of which are indicated at 240. There are eight bits (241) defining an operation code (OP CODE), four hits 242 designating a first local register R in CE, and four bits 243 designating a second local register R in CB. Not shown at 240 are two parity bits indicating the correct parity of the other sixteen bits.

The registers denoted by R and R are the registers 195 and 196 shown in FIG. 11. These are preset under program control with respective 36-bit words representing a configuration mask (CM) and selection mask (SM) appropriate to the selected SCON instruction. Such loading may take place as part of step 229 or it may have 14 been effected at some previous time as part of a program process of collecting several CM and SM words in several storage locations or registers.

During this phase of the process the SC bits (SC in the CM word in R are checked, to make sure that they define a legal configuration, by OR circuit 185 and AND circuit 176 of FIG. 11. coincidentally, a current SC bit and status bits S S in CCR/CE, the SC bits in CM, and the current state of the CE test switch (165, FIG. 11) are checked by AND circuits 187 and 170, OR circuit 186, and AND circuit 176 (all FIG. 11) to verify that the CE is in a state in which it is permitted to execute SCON as discussed hereafter. If all checks are successful SCON latch (FIG. 11) is set, and simultaneously (231) the CM intelligence accompanied by SM selecting bits is transferred out via appropriate buses (extensions of 71, 73, FIG. ll) and presented selectively to the individual CCR receiving gates of the complex. As each element response is received (232) via bus 250, FIG. 11, the corresponding SM selecting bit in R is reset. Hence, execution of the SCON instruction may be considered complete when all bits in R are set to 0. By storing the SM word in another register, CE can update the CCR status bits of the responding elc ments in the above-mentioned status table (step 233).

At this point it is necessary to determine whether the system has been completely reconfigured or only partially reconfigured. Hence, Ii is necessary to revert CF. to state 229 (FIG. 14) and to again conditionally exit, or proceed to another SCON instruction 231. Eventually, after one or more passes through loop 229-233 an exit condition is reached terminating the configuring program.

The status table shown in FIG. 16 includes a pair of current status bits, S S for each element of the complex, the values of which correspond to conditions stored in the status bit stages of respective element CCRs. To condition program operation 229 on the states of these hits it is necessary for the programmer to keep in mind the desired end configuration of CCR bits and to program towards that end.

A word of explanation is in order at this point. In the exemplary embodiment under consideration there are five distinct states shown in the table of FIG. 15, and a programming convention used in the exemplary present organization is that all elements in the same state will receive only from each other. While this is not an essential restriction it simplifies the reconfiguration program considerably since the configuration mask (CM) for any element can be composed by entering ones in the CM bit positions assigned to other elements in the same state and zeros in all other bit positions. Without this restriction it would be necessary to program more of the (CR information into the status table of FIG. 16.

Now let us consider an example as follows. Assume that CB CB SE I0 and all adapters except TCU are in ACTIVE states with their respective pairs of status bits set to the condition 11" (note FIG. 16). This means that all of the real time processing is being handled by these elements, that each element in this group can re ceive intelligence from any other element of the group to which it has a data path connection, due to corresponding 1 bits in the CCR's, and that each element in the group is insensitive, by virtue of 0 bits in its CCR. to communications from elements not in the group. Now assume CB has failed and it is desired to reassign (E from the ACTIVE group to the TEST group while bringing in CE presently in REDUNDANT state (8 :1, 5 :0), as a replacement for CE. This may be accomplished by means of two SCON instructions executed in two passes through loop 229-233 of FIG. 14, as follows:

CE may execute the reconfiguration program.

15 The CM word in register R of CE in the first csccution of step 231 [Ht]. 14 would have the form:

. S L s so P 1: P2 51: cr P3 to 1:

loiro The SM word in register R of (F would have the This would reorganize the ACTtVE group (S d. 5 :1) to include CB CE SE i I and all adapters except TCU It is yet necessary. ho .ve\er. to move CE from the active to the TEST group {51:9, S -"0) so fir liiiiiooo noon PAM 'ICU E SE 8 SE l iiipiiK l 11111000 that its fault may be diagnosed. For this a second pro SC 1? SE CE g inni V And the SM word for correctly distributing this CM throughout the complex would be:

3(3901001 'JlO PAM row se P SE or: P to P 2 3 4 l M91100 //1 00000011 oneoioni oin,

Assuming that the fault in CH is not located in CCR CE or its access network. the above (M wiIl be correctly accepted by (E and the reconfiguration proc ess may then be terminated. It might be mentioned that this assumption is a realistic one since this sma l segment of CH will rarely fail. Upon termination of icconliguu'ation CE will be ready to undergo manual tests or automatic tests under program control ol CH When CE has been repaired it will usually be desirable to reconfigure CE; or CE into the REDUNDANT group.

The question of priority relating to simultaneous requests for service by several elements was briefly touched upon above. It is worthwile to note that IO and units will generaily not impose significant conflicting request loads on an SE since the IO elements act in re sponse to I/O instructions executed by the Cii's. A priority resolution problem also occurs when several CE's attempt to share access to one SE simultaneously or in interleaved passes. This is a problem only when a CE is executing an extended program involving many refcrcnces to SE. for example a data sorting program. It is not a problem for the procedure involved in FIG. 14.

Nevertheless it is noted for the sake of completeness that hardware is provided in each SE for selecting a unique access request from among any lurality of copending requests.

It may be observed that in the example above correspondingly ordered SC and CE bits in the second CM word do not have equal values. This results from a specialized programming restriction which is stated as follows: A CM issued to form an ACTIVE or RE- DUNDANT group will have SC bits set to l in the positions corresponding to all ACTIVE and REDUN- DANT CEs (this permits a REDUNDANT CE upon receipt of ELC from a CF. to reconfigure the complex by iirst resetting the CCR state hits of the receiving (E to the ACTIVE state ill) as indicated in Note 4 lil] (FIG. 15)). A CM iusued to form a TEST group may contain 1s in SC bit positions assigned to CEs in the TEST state as well as in other positions to permit recall of TEST elements to other states by CEs in other states. The question occurs, what happens when an additional non-failing element is required in the TEST group. The answer is that only a CE in the ACTIVE group could safely make such a reassignment since only such a CE can be presumed to be aware of current processing needs. It is advisable to add that an element in TEST state may be less available" to the ACTIVE system than an element in REDUNDANT state since it may be accumulating important test data in a manner not susceptible to interruption, whereas processing in the REDUN- DANT group is of the type which is considered instantaneously interruptable; for example, program debugging routines.

It should be obvious that if all CCR bits were stored with the status table of FIG. 16 considerably more sophisticated reconfiguration schedules could be followed. However, the present plan of organization simplifies the reconfiguration programming etfort by limiting the number of passes through the reconfiguration subroutine to at most four tie, the number of distinct states defined by the CCR status bits: ACTIVE (ll), REDUNDANT (01), REDUNDANT (l0), and TEST (00) and is adequate for most multiprocessing purposes).

CCR/SE Referring to FIG. 12, CCR/SE is a register 300 in which the twelve SE bit positions are unused since one SE cannot communicate with other SEs. The groups of AND circuits 3tl53t)8 controlled by outputs of respective AND circuits 301-304 are adapted to selectively translate respective CM words CM/CE CM/CE CM/CE and (TM/(7E to OR circuits 315. AND circuits 301-304 are individually controlled by SE; bits in SM words issued by CE A respectively, and by bits 8C SC SC and SC; in CCR/SE (register 300). AND circuits 3(11-304 are further jointly controlled by 3 TEST switch 316 acting through 2: TEST LATCH 317. With latch 317 OFF circuits 301-304 are each partially enabled. Latch 317 is set OFF when switch 316 is OFF and ON when AND circuit 318 responds to the occurrence of: TEST switch 316 ON and CCR/SE /bit s CCR/sE /bit 5 :0.

Manually keyed CCR changes are entered in CCR/ SE via AND's 327, corresponding functionally to ANDs 127 of FIG. 10. OR 333 and bypass circuit 337 perform functions corresponding to those respectively provided by OH 133 and bypass circuit 137 of FIG. 10. Check circuits not shown provide the parity and other checks relevant to issuance of ELC/SE (on a line also not shown). OR circuit 350 conveys SCON signals from CE 3 4 via line 320 to ANDs 381-304.

As suggested in FIG. 13, the CCR circuits for a representative IO are substantially similar to the CCR/SE circuits of FIG. 12, except that the IO bit locations in CCR/IO are unused. This is because IOs need not exchange information betwcen each other in the present organization. Of course, this is not an essential restriction since it may be desirable in some complexes to permit transfers between IOs.

Referring to FIG. 15, in the subject organization an element can be established in any of five distinct states: ACTIVE (8 :1, S i), REDUNDANT (S =0, S zl), REDUNDANT (5 :1, 5 :0), TEST WITH LATCH OFF (S O, S :O), and TEST WITH LATCH ON (83:0, S :0). REDUNDANTJO differs from RE- DUNDANT-til only in that the former state affords some access for manual control while the latter does not. CEs in either REDUNDANT state may not issue SCON, but may accept SCON from other CEs. TEST WITH LATCH OFF is a state which permits 11 CE to issue SCON externally (to other elements in the same 

